The present invention relates to an oscillator having a tunable resonant circuit which contains a tuning element, for example a variable capacitance diode, and a switch for expanding the tuning range. The switch permits one of the components which influence the oscillation frequency, usually a coil, to be bridged, with the result that this causes a changeover to another frequency band. Circuits of this type are used, in particular, in television sets to receive the lower and upper VHF bands.
A known circuit of this type is explained in more detail with the aid of FIG. 1: connected to an integrated circuit IC1 is a tunable LC resonant circuit having frequency-influencing components, coils L1 and L2 and capacitances C4 and C5. The integrated circuit IC1 is a commercially available tuner IC, for example of the type TUA 6014 XS from Siemens AG, which, besides the oscillator, also contains further circuits, such as a mixer and a PLL, and processes both the UHF band and the VHF band. In addition to various control voltages, the radio-frequency signal V (VHF) is also applied to this circuit.
The LC resonant circuit is arranged in the form of a parallel resonant circuit on the integrated circuit IC1, with the two coils L1 and L2 and the two capacitances C4 and C5 being connected in series in each case. One of the two capacitances, C4, is a variable capacitance diode, whose capacitance can be varied by a tuning voltage UT. In this arrangement, this voltage UT is applied to the variable capacitance diode C4 and to the resonant circuit via two relatively high-value resistors R3 and R4. Capacitors Ci1-Ci4 are used to couple the resonant circuit to the integrated circuit IC1.
A switch SD, usually a PIN diode, can be used to bridge one of the two coils, in this case the coil L2, which permits the oscillation frequency to be shifted through a relatively large range. In this context, turning on the switch SD connects the coil L1 to the capacitor C4, so that the inductance of the coil L2 is of no significance for the oscillation frequency in this case. To turn on the switch SD, a switching voltage US is required, which is applied to the LC resonant circuit via a resistor RS1 and is dissipated to earth G downstream of the switch SD via a resistor RS2. The capacitors C2 and C3, which act as a short circuit for radio-frequency purposes, decouple the tuning voltage UT and the switching voltage US, and the capacitor C1 filters the switching voltage US.
To change the switch SD to a low-resistance state, an appropriate current of a plurality of milliamperes needs to flow, depending on the type of switching diode SD. The result of this is that, depending on the available switching voltage US, e.g. 0 V and 5 V, the resistors RS1 and RS2 are in the region of one kilohm or less. Consequently, in the xe2x80x9cupper VHF bandxe2x80x9d switching state, in which the switching diode SD is on, the oscillator is asymmetrically loaded by the parallel circuit comprising the resistors RS1 and RS2. In the xe2x80x9clower VHF bandxe2x80x9d switching state, when the switch SD is off, the coil L2 is resistively damped by the series circuit comprising the resistors RS1 and RS2 which is now present. The result is an impaired circuit quality factor, and hence impairment of the oscillation properties, which may result in oscillation suppression. Furthermore, the phase noise properties are impaired on account of the impaired circuit quality factor. Oscillators having good phase noise properties are required particularly for digital transmission methods, however.
The relatively low-value resistors RS1 and RS2 cannot be chosen to have higher resistance values, because the current through the switch SD becomes too small otherwise. These resistors could be replaced with inductors whose inductance value is much higher than the circuit coil inductance, but these are many times more expensive than the resistors and have natural resonance points which can likewise have an adverse effect on the oscillation properties of the oscillator.
The object of the invention is therefore to specify an economical oscillator having a tuning range which can be changed over using resistors and having good oscillation reliability and good phase noise properties.
The oscillator having a tunable resonant circuit in accordance with the invention has a switch for expanding the tuning range, which is connected in series with two coils jointly determining the oscillation frequency of the resonant circuit. A further coil, which is coupled to the resonant circuit in the region of the switch, is used to supply a switching voltage for the switch. In this arrangement, the other end of this third coil is coupled to a reference potential, for example earth, with a capacitor, which acts as a short circuit for radio-frequency purposes, decoupling the switching voltage from the reference potential.
Coupled to the other end of the switching diode is a further component, preferably a fourth coil, which has the same inductance as the third coil, in order to dissipate the switching voltage. The operation of the oscillator circuit in this case is as follows: if the switch is on, the third coil and the further components, connected to the switch SD in the context of the switching voltage, are of no significance for the oscillation frequency. If the switch is off, the third coil, just like the fourth coil, acts as a frequency-influencing element for the resonant circuit. Since the switching voltage is in this case coupled to an oscillator circuit node which is earthed for radio-frequency purposes, it is of no significance in this case which resistor is used to couple the switching voltage. This means that the resonant circuit is not resistively damped by the connection of the switching voltage.
The first two coils are advantageously of the same size, so that this circuit coil jointly determining the oscillation frequency is earthed by the third coil exactly at its centre. This means that the quality factor and the oscillation properties of the resonant circuit are not adversely affected.
This arrangement of the switch permits the resonant circuit to be of strictly symmetrical design when the fourth coil is used, because the third coil is coupled to the reference potential or to earth via the capacitor for radio-frequency purposes. Since the third and fourth coils can be in the form of air-core coils, the circuit may be of very economical design. No expensive inductors are used for changeover purposes.